Optical fiber has been used in telecommunication networks as “backbones” to link central offices (COs) to one another. Traditionally, ordinary twisted pairs of copper wire (“wire-pairs”) have been used as “local loops” to link the COs to a customer's premises (CP). Wire-pairs, however, are not conducive to providing high data rate services to customers over the entire distance between a CO and the CP. Accordingly, telecommunication companies are transitioning to using optical fiber between the COs and the customer premises.
Fiber-to-the-Curb/Node/Premises (FTTC/FTTN/FTTP or, generally, FTTX) is a distribution method for delivering a combination of broadband services to customers. FTTX offers a network architecture that uses optical fiber to replace all or part of the wire-pairs between the CO and the CP to deliver a combination of broadband services including voice, Internet, broadcast television and video-on-demand. An interconnection point is used at some point between the CO and the CP to connect the optical fiber to the wire-pairs. The interconnection point may be, for example, a connection box or a node.
At an interconnection point, conversion equipment is needed to convert the optical signals on the optical fiber to electrical signals for delivery to the CP over the wire-pair. This requires the location of power consuming equipment at the interconnection point and necessarily outside of the battery-backed CO environment.
Unfortunately, optical fibers are unsuited to convey the electric power from the CO to the conversion equipment. The conversion equipment, therefore, must derive its electric power in a different way. The conversion equipment could use the commercial power grid at the interconnection point. This, however, requires a connection to the power grid and is unsatisfactory when the power grid fails. The conversion equipment loses its source of power, and telecommunication services are interrupted until power is restored. Given the historical reliability of the nation's telecommunication system, this is unacceptable.
One way to deliver power to remote sites without interruptions is to use uninterruptible power delivered from the CO at elevated voltages over wire-pairs. Thus, the wire-pairs that the optical fibers superseded can be used as conduits for electric power rather than for telephone conversations. The wire-pairs can provide power all the time or only during grid failures. Historically, a power converter is used to convert the operating voltage at the CO to a higher voltage for transmission across the wire-pairs. The transmission voltage is then converted to a suitable operating voltage for the equipment at the interconnection point. Typically, the CO operating voltage is 48 V DC and power is delivered to the equipment via the wire-pairs via a −190 output and a return. Multiple wire-pairs in parallel may be used.
Although the latter approach avoids connecting to the commercial power grid, using wire-pairs for electric power has its own complications. The relatively high transmission voltage of 190 V DC is necessary given the electrical resistance of the wire-pairs. However, to maintain safety, all wire-pairs used for local loops fall under Class 2 of the National Electrical Code (NEC), which establishes a 100 volt-ampere (VA) limit on the wire-pairs. Therefore, line-dedicated 190V current limiters must also be used at the CO to ensure that the 100 VA limit per twisted pair is not exceeded. Additionally, when the transmission voltage exceeds 140 V DC, the ground fault current can not exceed 10 mA even in the case where one of the wires of the wire-pair (the output or return) is connected or shorted to Earth. This limit is a safety requirement mandated by, for example, Underwriters Laboratories standard UL-60950-21, Canadian Standards Association CSA-22.2 and Network Equipment-Building System (NEBS) GR1089-CORE. Accordingly, what is needed in the art is a system and method to provide dependable power to conversion equipment located distal from the CO while complying with the mandated safety requirements.